Odoo is not a mind reader, you would need to manually choose the correct BOM before you confirm the manufacturing order.  If you have several BOM, you can create one without components, and set it on top of the list of BOM's. Then it will be the default one.  Odoo do not allow a manufacturing order to be without components, so the manufacturing order will stay in draft mode, and you can manually change the BOM and confirm the order.

More automatic: Use variants for the finish product (sales product), and different BOM for each variant.  But I don't think that will be efficient for you.

My choose for a situation like that would be to say that Corn is Corn, regardless of its origin.  So just one and the same product. Then set traceability to lot number, both for corn and the finish product. Register what lot numbers are used in each manufacturing order.  Labels with barcode is very usefull is such a flow. 

From traceablity you will know the origin.

Set pricing to FIFO. Both for the corn and the finish product. You will have cost control.